1. Field of the Invention
The present invention relates to a fine fiber-reinforced thermoplastic elastomer composition, a process for producing the thermoplastic elastomer composition, a fine fiber-reinforced elastic material and a process for producing the elastic material.
More particularly, the present invention relates to a fine fiber-reinforced thermoplastic elastomer composition in which a matrix comprising an elastic component and a polyolefin component is reinforced by a fine fibrous polyamide component, a process for producing the thermoplastic elastomer composition at a high efficiency, a fine fiber-reinforced elastic material comprising the thermoplastic elastomer composition and an additional elastic component, having excellent elastic modulus, tensile strength, and processability, and a process for producing the elastic material.
The fine fiber-reinforced elastomer composition and elastic material are useful for producing outside parts, for example, tread and side wall, of a tire, inside parts, for example, a carcass, braded edge, belt and chafer, of a tire, and industrial products, for example, hoses, belts, rubber rolls, and rubber crawlers.
2. Description of the Related Art
It is known that polyolefin resin materials, for example, polypropylene and ethylene-propylene copolymer materials, are widely used for forming bumpers and trim of motorcars and casings and parts of home electric appliances.
In most of these polyolefin resin materials, an elastic polymer, for example, EPDM (ethylene-propylene-diene terpolymer) is blended to enhance the impact strength thereof.
For example, Japanese Unexamined Patent Publication (Kokai) No. 01-104,636 discloses a polymer composition comprising a polypropylene resin and an ethylene-propylene copolymeric rubber which are graft-bonded to an organic short fibrous material through a coupling agent.
Also, Japanese Examined Patent Publication (Kokoku) No. 02-248,448 discloses a thermoplastic resin composition comprising a thermoplastic resin and a mixture of a chlorinated polyethylene and short fibrous polyamide.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2-251,550 discloses a fiber-reinforced composition comprising chlorinated polyethylene and a polyamide in the form of fine fiber and dispersed in the chlorinated polyethylene.
Conventional fiber-reinforced elastomeric materials comprise a composition comprising short fibers dispersed in a vulcanizable rubber material, for example, natural rubber polyisoprene, polybutadiene rubber or ethylene-propylene copolymeric rubbers, to enhance the elastic modulus and mechanical strength thereof. The conventional fiber-reinforced elastomeric materials are produced by blending an elastomeric material with short fibers comprising nylon, polyester or water-insolubilized polyvinyl alcohol, and optionally vulcanizing the blend.
The conventional fiber-reinforced elastomeric materials produced by the above-mentioned method are unsatisfactory in mechanical strength and elongation when used as a material for forming tires of motorcars. Accordingly, there is a demand for a new fiber-reinforced elastomeric material having an enhanced mechanical strength and elongation.
In response to this demand, there have been made attempts to provide an improved elastomeric material reinforced by using fine short fibers, for example, nylon fibers, having an average thickness of less than one .mu.m. This type of fiber-reinforced elastomeric material can be produced by melt-kneading a mixture of a vulcanizable rubber material, a nylon resin and a binding agent at a temperature equal to or higher than the melting temperature of the nylon resin, extruding the kneaded melt through an extruding die at a temperature equal to or higher than the melting temperature of the nylon resin to form a thread, and drawing or press-rolling the thread. Optionally, the mixture is further admixed with a vulcanizing agent, and the drawn or press-rolled material is vulcanized. This type of process is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-79037 in which a precondensate of a resol type alkyl phenol-formaldehyde resin is used as a binding agent, or Japanese Unexamined Patent Publication (Kokai) No. 59-43,041 in which a precondensate of a novolak type alkylphenol-formaldehyde resin is used as a binding agent, or in Japanese Unexamined Patent Publication (Kokai) No. 63-81,137 in which a silane coupling agent is employed as a binding agent.
In the conventional elastomeric materials, it is known that when an elastic polymer is blended into the polyolefin resin, the resultant blend material exhibits a reduced rigidity and mechanical strength, a decreased stress at yield and a decreased creep resistance. To prevent the reduction in the rigidity, mechanical strength, stress at yield and creep resistance, glass fibers and/or an inorganic filler is blended, in addition to the elastic polymer, into the polyolefin resin. However, an increase in the content of the glass fibers or the inorganic filler causes the resultant shaped article to exhibit an unpleasant appearance and an increased specific gravity. Generally, the conventional fiber-reinforced elastomer compositions are advantageous in not only having an excellent tensile strength and elastic modulus, but also in having a high tear strength and a high elongation. However, the conventional fiber-reinforced elastomer composition is disadvantageous in that since the fine fibers, for example, fine nylon fibers, are dispersed in an elastic matrix, the resultant composition is difficult to pelletize and thus can be produced only in the form of a sheet or lumps. Also, the conventional fiber-reinforced elastomer composition is disadvantageous in that when the reinforcing fibers, for example, nylon fibers are mixed in a large amount, the resultant composition becomes too stiff.
Due to the above disadvantages, the conventional elastomer composition must be cut by hand when it is admixed with additional vulcanizable elastic material and/or vulcanizing agent. Also, sometimes, the reinforcing fibers and the additional vulcanizable elastic material are difficult to evenly disperse in the elastomer composition. To prevent these disadvantages, it is necessary to prolong the melt-kneading time or to admix additional vulcanizable elastomeric material or other additive into the elastomer composition after softening the elastomer composition by heating. This necessity results in a decrease in productivity of the fiber-reinforced elastomer composition and in increase in production cost.
The conventional fiber-reinforced elastomer composition is advantageous in having a high elastic modulus and fatigue resistance in the orientation direction of the reinforcing fibers. However, the conventional fiber-reinforced elastomer composition is disadvantageous in that the elastic modulus and fatigue resistance in a direction at a right angle to the orientation direction of the reinforcing fibers in the composition are significantly low.